Back to EveryPatent.com
United States Patent |
5,105,645
|
Kobayashi
,   et al.
|
April 21, 1992
|
Method of redrawing metal cup
Abstract
Disclosed is a method of redrawing a metal cup, in which an annular working
member is used together with a redrawing die, a redrawing punch and a
cup-shaped holding member, and this annular working member has a working
face having an inner diameter smaller than the outer diameter of a
preliminarily drawn cup to be redrawn. If redrawing is carried out by
using this annular working member, the thickness of the drawn cup can be
uniformly and sufficiently reduced without damaging a metal sheet of the
preliminarily drawn cup or a coating layer thereof.
Inventors:
|
Kobayashi; Tomomi (Yokohama, JP);
Kobayashi; Akira (Yokohama, JP);
Imazu; Katsuhiro (Yokohama, JP)
|
Assignee:
|
Toyo Seikan Kaisha, Ltd. (Tokyo, JP)
|
Appl. No.:
|
611636 |
Filed:
|
November 13, 1990 |
Foreign Application Priority Data
Current U.S. Class: |
72/348; 72/379.4 |
Intern'l Class: |
B21D 022/28 |
Field of Search: |
72/347,348,349,379.4
|
References Cited
U.S. Patent Documents
1942930 | Jan., 1934 | Ludington | 72/349.
|
4425778 | Jan., 1984 | Franek et al. | 72/349.
|
Primary Examiner: Larson; Lowell A.
Attorney, Agent or Firm: Sughrue, Mion, Zinn, Macpeak & Seas
Claims
We claim:
1. In a method for redrawing a metal cup, which comprises holding a
preliminary drawn cup of a metal sheet by a cup-holding member inserted in
the cup and a flat face portion of a redrawing die, and relatively moving
the redrawing die and a redrawing punch arranged coaxially with the
holding member and the redrawing die and movably within the holding
member, an improvement wherein an annular working member coaxial with the
cup-holding member, which has a working face having an inner diameter
smaller than the outer diameter of the side wall of the preliminarily
drawn cup, is arranged on the introduction side of the flat face portion
of the redrawing die, and wherein the outer surface of the side wall of
the preliminarily drawn cup is engaged with the working face of the
annular member while maintaining the inner surface of the side wall of the
preliminarily drawn cup in a state free from the cup-holding member,
thereby to diminish the outer diameter of the preliminarily drawn cup by 1
to 6% prior to redraw forming, and subsequently, redraw forming of the cup
by the redrawing die and the redrawing punch while a blank holding force
is applied to the cup by the holding member and the flat face portion of
the redrawing die.
2. A redrawing method according to claim 1, wherein the outer diameter of
the preliminarily drawn cup is diminished by 1 to 5% by the annular
working member.
3. A redrawing method according to claim 1, wherein the radius, on the side
of the annular working member, of the holding member is 3 to 20 times the
thickness of the metal sheet.
4. A redrawing method according to claim 1, wherein the radius of a corner
portion having a tensile bending action in the redrawing die is 1 to 20
times the thickness of the metal sheet.
5. A redrawing method according to claim 1, wherein the metal sheet is a
coated metal sheet having a protecting resin coating.
Description
BACKGROUND OF THE INVENTION
(1) Field of the Invention
The present invention relates to a method of redrawing a metal cup. More
particularly, the present invention relates to a redrawing method of
preparing a uniformly thickness-reduced can barrel from a preliminarily
drawn cup of a metal sheet, especially a coated metal sheet, while
moderating the damage of the metal sheet and the coating layer and
controlling the advance of the work hardening.
(2) Description of the Related Art
The production of a seamless can barrel by subjecting a metal sheet or a
coated metal sheet to drawing and redrawing has been conducted from old.
At the draw-redraw forming, the metal sheet shows such a plastic flow that
the size of the can is increased in the height direction but the size is
diminished in the circumferential direction of the can barrel.
According to the conventional redrawing method, the redraw forming is
accomplished by relatively moving a punch having a diameter smaller than
that of a preliminarily drawn cup drawn in advance to have a large
diameter and a redrawing die so that the punch and die are engaged with
each other. An annular cup-holding member is arranged within the
preliminarily drawn cup, and a bottom face of the preliminarily drawn cup
is held by the holding member and the flat face portion of the redrawing
die. At the redrawing step, the holding member is moved synchronously with
the redrawing die.
In this structure, at the relative movement of the punch and the die, the
preliminarily drawn cup is draw-formed into a deep-draw-formed cup having
a small diameter by a curvature corner portion of the redrawing die and
simultaneously, the side wall of the preliminarily drawn cup is bent and
elongated and the thickness of the side wall is reduced. At the deep-draw
forming, the holding member and the flat face portion of the redrawing die
act as the blank holding face to a portion of the cup on which influences
of the plastic flow are imposed.
Various methods of sufficiently reducing the thickness of a
deep-draw-formed cup and uniformly reducing the thickness of the side wall
of the cup have been proposed in the conventional thickness-reducing
redrawing process (see, for example, Japanese Unexamined Patent
Publication No. 56-501442 and Japanese Unexamined Patent Publication No.
01-258822).
These thickness-reducing redrawing methods are characterized in that the
curvature of the corner portion of the redrawing die is specifically
adjusted, and by reducing the curvature of the corner portion, the
thickness of the deep-draw-formed cup is reduced uniformly and
sufficiently. However, if the curvature of the corner portion of the
redrawing die is reduced, the risk of damage of the metal sheet and
coating layer increases and the adhesion between the coating layer and the
metal sheet tends to decrease. Accordingly, in a final canned product,
troubles such as corrosion of the metal and elution of the metal are
caused, and a swollen can is formed by generation of hydrogen or a leaking
can is formed by pitting.
In order to obtain a large thickness reduction ratio (1--can barrel
thickness/blank thickness), it is necessary to set the radius of the
redrawing die and the radius R.sub.H of the holding member at smaller
values or to apply a larger blank holding force. However, in the case
where the surface of the material is coated, the bearing pressure imposed
on the coating surface rises and it is apprehended that the coating will
be fatally damaged. Furthermore, in the case where the side wall face of
the preliminarily drawn cup is arranged on the blank holding face, the
space from the blank holding face is equal to the maximum sheet thickness
in the blank holding face, and therefore, if a cup having an uneven
thickness distribution is redrawn, the blank holding force is concentrated
on the peripheral side of the blank holding face where the thickness is
largest, while no satisfactory blank holding effect is obtained on the
inner circumferential side of the blank holding face, and traces of
wrinkles are sometimes left on the redraw-formed cup.
In the case where a draw-redraw-formed cup is processed in a vessel for a
canned product, the cup is then subjected to beading, necking and flanging
of the side wall. In general, in the side wall of the draw-redraw-formed
can barrel, work hardening is caused by plastic deformation and the yield
point becomes larger than that of the starting blank, and therefore, there
is observed a tendency that the above-mentioned post treatments are not
advantageously performed. This work hardening is especially conspicuous in
the vicinity of the open end of a cup obtained from a high-strength steel
sheet as the starting blank at high draw and redraw ratios, and the
above-mentioned undesirable tendency is prominent in this cup.
SUMMARY OF THE INVENTION
It is therefore a primary object of the present invention to provide a
redrawing method of producing a deep-draw-formed can, in which the
thickness is uniformly and sufficiently reduced even if the curvature
radius of the redrawing die and the curvature radius (blank holder radius)
of the corner portion of the holding member are set at larger values.
Another object of the present invention is to provide a redrawing method in
which damage of a metal sheet or coating is reduced.
Still another object of the present invention is to provide a redrawing
method in which work hardening is controlled in the redraw-formed cup and
post treatments of the redraw-formed cup, such as necking, are
facilitated.
In accordance with the present invention, there is provided a method of
redrawing a metal cup, which comprises holding a preliminarily drawn cup
of a metal sheet by a cup-holding member inserted in the cup and a flat
face portion of a redrawing die, and relatively moving the redrawing die
and a redrawing punch arranged coaxially with the holding member and the
redrawing die and movably within the holding member, wherein an annularly
working member coaxial with the cup-holding member, which has a working
face having an inner diameter smaller than the outer diameter of the side
wall of the preliminary drawn cup, is arranged on the introduction side of
the flat face portion of the redrawing die, the preliminarily drawn cup is
passed through the working face of the annular working member to diminish
the outer diameter of the cup, and subsequently, redraw forming of the cup
is carried out by the redrawing die and the redrawing punch while a blank
holding force is being applied to the cup by the holding member and the
flat face portion of the redrawing die.
In the present invention, it is especially preferred that the outer
diameter of the preliminarily drawn cup is diminished by 1 to 6% by the
annular working member.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1 and 2 are sectional views showing the main part, which illustrate
the redrawing method of the present invention.
FIGS. 3-A and 3-B are sectional partial views illustrating the problems to
be solved.
FIG. 4 is a sectional partial view illustrating an embodiment in which the
method of the present invention is worked as the reverse redraw-forming
method.
FIG. 5 is a diagram illustrating an example of the material used in the
present invention.
FIG. 6 is a diagram illustrating the drawing and redrawing process.
FIG. 7 is a diagram illustrating Comparative Example 1 given hereinafter.
DETAILED DESCRIPTION OF THE INVENTION
The redrawing method of the present invention will now be described.
Referring to FIG. 1, a redrawing punch 1, a redrawing die 2, a holding
member 3 and an annular working member 4 are used in the redrawing method.
The redrawing punch 1 is arranged so that the redrawing punch can be moved
relatively to the redrawing die 3 and holding member 3, and the redrawing
die 2 and annular holding member 3 are arranged with a certain space
therebetween according to the thickness of a preliminarily drawn cup (cup
drawn at the preceding step) 5. The annular working member 4 is arranged
on the introduction side of the redrawing die 2. In the redrawing die 2,
the flat face portion of the redrawing die constitutes a top end face 2A
of the redrawing die 2, and in the holding member 3, a small-diameter
lower end face 3A of the holding member constitutes a blank holding face.
In the annular working member 4, a working portion 4A is formed on the
inner wall of the working member 4, and the diameter of the working
portion is smaller than the outer diameter of the side wall of the
preliminarily drawn cup to be used.
In the present invention, a working face having a diameter smaller than the
diameter of the preliminarily drawn cup is disposed on the introduction
side of the flat portion of the redrawing die, and at the redrawing step,
at first, the preliminarily drawn cup is passed within the working face to
diminish the outer diameter of the preliminarily drawn cup and then,
redraw forming is carried out while a blank holding force is being applied
by the holding member and the flat face of the redrawing die, whereby
damage of the metal sheet or coating layer of the redraw-formed cup is
drastically reduced.
Furthermore, in the present invention, prior to redraw forming, the
diameter of the preliminarily drawn cup is diminished by the working face
and the hardness of the portion to be processed is reduced, and therefore,
the post processing of the redrawn cup is greatly facilitated.
In the present invention, at first, the side wall of the preliminarily
drawn cup undergoes a working action of the working portion of the annular
working member, with the result that the diameter of the preliminarily
drawn cup is diminished. At this point, the side wall of the cup undergoes
a bending-unbending action of a curved part of the large-diameter portion
of the holding member and a working portion of the annular member. Then,
the side wall of the cup, which has undergone such actions, receives a
blank holding force from a small-diameter corner portion 3C of the holding
member, and also receives a bending resistance when the side wall of the
cup is introduced into the clearance of the blank holding face.
Furthermore, when the side wall of the cup receives this bending
resistance, a tension from the working face of the preceding stage is
imposed. After the side wall of the cup has passed through the blank
holding face, the side wall of the cup is bent and elongated by a corner
portion 2B of the redrawing die. Also in this case, a back tension is
applied to the side wall at the corner portion 2B.
In the above-mentioned structure, at first, the diameter of the
preliminarily drawn cup is diminished. Accordingly, the difference of the
thickness of the cup between the peripheral side and inner circumferential
side of the blank holding face is reduced, and the blank holding force
acts effectively. Furthermore, also the area of the blank holding face is
reduced and the blank holding force can be reduced.
Furthermore, in the present invention, the holding member is inserted into
the preliminarily drawn cup, and when the cup receives a pressing force
from a small-diameter corner portion 4C of the holding member, the
peripheral portion of the bottom of the preliminarily drawn cup has
already received a compression deformation in the circumferential
direction from the working portion 4A of the annular member. In the
conventional technique, the optimum range of the punch radius R.sub.P at
the forming of the preliminarily drawn cup is different from the optimum
range of the radius R.sub.H (blank holder radius) of the corner portion 3C
of the holding member and the relation of R.sub.P >R.sub.H is established.
When the holding member is inserted in the preliminarily drawn cup at the
redrawing step and the blank holding force acts on the cup, as shown in
FIGS. 3-A and 3-B, if the peripheral curved portion of the bottom of the
preliminarily drawn cup is expanded outwardly in the radial direction of
the cup, because of the above-mentioned difference of the optimum range, a
tensile deformation is caused in the above portion in the circumferential
direction, with the result that local reduction of the thickness is
caused. If a coating is formed in advance on the inner face or outer face
of the metal sheet, it is apprehended that damages such as cracks will be
formed on the coating. In contrast, according to the present invention,
even if there is a difference between R.sub.P and R.sub.H, since a
compression deformation as mentioned above is caused on the working face
4a, the radius R.sub.H of the corner portion 3C of the holding member can
be set within the optimum range even under severe conditions imposed on
the preliminarily drawn cup. Furthermore, the local reduction of the
thickness and the sequential damage of the coating can be effectively
avoided. In the present invention, it is preferred that the radius R.sub.H
of the corner portion of the holding member be 3 to 20 times, especially 4
to 12 times, as large as the sheet thickness.
If R.sub.H is too small and below the above-mentioned range, the bending
resistance imposed when the material is introduced into the clearance of
the blank holding face becomes large, and fracture of the material is
often caused. If R.sub.H is too large, the quantity of an uncontrollable
deformation of this portion becomes large and wrinkles are easily formed.
In case of thickness-reducing redraw forming where the thickness of the
side wall of the can barrel is uniformly reduced by application of not
only a bending-unbending deformation but also a tensile stress at the
radius portion of the die, it is especially important to set R.sub.H at an
appropriate value, and it is preferred that R.sub.H be adjusted to a
relatively small value within a range not causing breaking of the barrel,
that is, 4 to 12 times the sheet thickness.
Furthermore, in the state where the side wall of the cup receives back
tensions from the large-diameter terminal corner portion 3B of the holding
member, the working face 4A of the annular member and the small-diameter
terminal corner portion 3C of the holding member, the side wall of the cup
undergoes a tensile bending deformation at the corner portion 2A of the
redrawing die. This co-operation of the back tensions exerts a functional
effect valuable for uniformly reducing the thickness of the cup. According
to the present invention, just before the material is introduced into the
clearance of the blank holding face, forming of diminishing the diameter
of the cup is performed by the annular member, and the
deformation-resisting force acting at this point acts effectively as the
back tension. In the case where in the present invention, it is intended
to obtain a thickness reduction ratio comparable to the thickness
reduction ratio attained in the conventional method, the radius R.sub.D of
the corner portion 2A of the redrawing die and the radius R.sub.H of the
corner portion 3C of the holding member can be made larger than in the
conventional method and the blank holding force can be set at a smaller
value than in the conventional method. Therefore, according to the present
invention, a cup having a drastically reduced damage can be obtained. It
also is important that the diameter of the working portion 4A of the
annular member should be smaller by 1 to 6%, especially 2 to 5%, than the
outer diameter of the preliminarily drawn cup. The diameter diminishment
ratio .delta. of the cup at the working portion of the annular member has
an optimum range. The radius diminishment ratio .delta. referred to herein
is a value defined by the following formula:
##EQU1##
If .delta. is too large, the tendency of formation of wrinkles increases,
and the deformation-resisting force in the annular member increases,
resulting in breaking of the barrel. If .delta. is too small, the intended
effect of the present invention cannot be sufficiently attained.
Therefore, in the present invention, the inner diameter of the annular
member is adjusted within the above-mentioned range.
Moreover, when the diameter of the preliminarily drawn cup is diminished by
the annular member, there also is present an optimum range for the angle
.theta. between the tapered portion of the cup and the central axis, and
if the angle .theta. is above or below this optimum range, forming
troubles such as wrinkling are caused. In the present invention, it is
preferred that the angle .theta. be in the range of from 15.degree. to
45.degree..
In the present invention, it also important that the side wall of the cup
should be subjected to bending-unbending at the large-diameter terminal
corner portion 3B of the holding member and the working portion 4B, as
pointed out hereinbefore. More specifically, in the preliminarily drawn
cup having the side wall where working hardening is caused, the hardness
is once reduced by the diminishment of the diameter by the annular working
member, and subsequently, while the cup is introduced into the clearance
of the blank holding face and is passed through the die radius portion,
work hardening is effected again. The hardness of the side wall of the
formed can barrel according to the present invention is lower than the
hardness of the cup formed into the same shape according to the
conventional redrawing method. Therefore, even in case of a highly
processed draw-redraw-formed can, according to the present invention, post
treatments such as bending, necking and flanging can be facilitated.
Moreover, the cup provided according to the present invention, the double
seaming operation for filling a content can be advantageously performed.
Incidentally, this functional effect is attained if the diameter
diminishment ratio of the cup at the working portion is within the range
of from 1 to 6%.
In the case where a relatively soft metal sheet or a relatively thick metal
sheet is used, the present invention can also be worked by using a holding
member 3' having neither a large-diameter portion 3D having a diameter
almost equal to or slightly smaller than the inner diameter of the
preliminarily drawn cup 5 nor a large-diameter terminal corner portion 3B,
as shown in FIG. 2. In this case, there is preferably disposed means for
guiding the preliminarily drawn cup at the point of the forming of the
preliminarily drawn cup so that the central axis of the preliminarily
drawn cup is in agreement with the central axes of the punch and the
holding member.
In the present invention, various surface-treated steel sheets and sheets
of light metals such as aluminum an be used as the metal sheet.
As the surface-treated steel sheet, there can be used steel sheets obtained
by annealing a cold-rolled steel sheet, subjecting the annealed steel
sheet to the secondary cold rolling and then subjecting the steel sheet to
at least one of surface treatments such as zinc plating, tin plating,
nickel plating, electrolytic chromate treatment and chromate treatment. As
a preferred example of the surface-treated steel sheet, there can be
mentioned an electrolytically chromate-treated steel sheet, especially one
having 10 to 200 mg/m.sup.2 of a metallic chromium layer and 1 to 50
mg/m.sup.2 (calculated as metallic chromium) of a chromium oxide layer.
This steel sheet is excellent in the combination of the coating adhesion
and corrosion resistance. As another example of the surface-treated steel
sheet, there can be mentioned a hard tinplate sheet having a tin
deposition amount of 0.5 to 11.2 g/m.sup.2. Preferably, this tinplate
sheet is subjected to the chromate treatment or the chromate/phosphate
treatment so that the chromium amount is 1 to 30 mg/m.sup.2 as metallic
chromium. As still another example, there can be mentioned an
aluminum-coated steel sheet formed by deposition or cladding aluminum.
Not only a so-called pure aluminum sheet but also an aluminum alloy sheet
can be used as the light metal sheet. An aluminum alloy sheet having a
high corrosion resistance and an excellent workability comprises 0.2 to
0.5% by weight of Mn, 0.8 to 5% by weight of Mg, 0.25 to 0.3% by weight of
Zn and 0.15 to 0.25% by weight of Cu, the balance being Al. Preferably,
these light metal sheets are subjected to the chromate treatment or the
chromate/phosphate treatment so that the chromium amount is 20 to 300
mg/m.sup.2 as metallic chromium.
The blank thickness (t.sub.B) of the metal sheet is changed according to
the kind of the metal and the use and size of the final vessel, but it is
generally preferred that the blank thickness be 0.10 to 0.50 mm,
especially 0.10 to 0.30 mm in case of a surface-treated steel sheet or
0.15 to 0.40 mm in case of a light metal sheet.
The present invention is advantageous in that if a protecting coating of a
resin is formed on a metal sheet prior to draw forming, deep-draw forming
and uniform reduction of the thickness can be performed without
substantial damage of the protecting covering layer. Formation of the
protecting coating can be accomplished by coating a protecting paint or
laminating a thermoplastic resin film on the metal sheet.
Protecting paints composed of thermosetting and thermoplastic resins can be
optionally used as the protecting paint. For example, there can be
mentioned modified epoxy paints such as a phenol-epoxy paint and an
amino-epoxy paint, vinyl and modified vinyl paints such as a vinyl
chloride/vinyl acetate copolymer paint, a partially saponified vinyl
chloride/vinyl acetate copolymer paint, a vinyl chloride/vinyl
acetate/maleic anhydride copolymer paint, an epoxy-modified vinyl paint,
an epoxyamino-modified vinyl paint and an epoxy-phenol-modified vinyl
paint, acrylic resin paints, and synthetic rubber paints such as a
styrene/butadiene copolymer paint. These paints can be used singly or in
the form of mixtures of two or more of them.
A paint as mentioned above is applied to a metal blank in the form of an
organic solvent solution such as an enamel or lacquer or in the form of an
aqueous dispersion or solution by roller coating, spray coating, dip
coating, electrostatic coating or electrophoretic deposition. In case of a
thermosetting resin paint, the coated paint is baked according to need. In
view of the corrosion resistance and workability, it is preferred that the
thickness (in the dry state) of the coating be 2 to 30 .mu.m, especially 3
to 20 .mu.m. A lubricant can be incorporated in the coating for improving
the draw-redraw formability.
As the thermosetting resin film used for the lamination, there can be
mentioned films of olefin resins such as polyethylene, polypropylene, an
ethylene/propylene copolymer, an ethylene/vinyl acetate copolymer, an
ethylene/acrylic ester copolymer and an ionomer, films of polyesters such
as polyethylene terephthalate, polybutylene terephthalate and an ethylene
terephthalate/isophthalate copolymer, films of polyamides such as nylon 6,
nylon 6,6, nylon 11 and nylon 12, and a polyvinylidene chloride film.
These films may be undrawn films or biaxially drawn films. Preferably, the
thickness of the thermoplastic resin film is 3 to 50 .mu.m, especially 5
to 40 .mu.m. Lamination of the film on the metal sheet can be accomplished
by heat fusion bonding, dry lamination, extrusion coating or the like. In
the case where the adhesiveness (heat fusion bondability) between the film
and the metal sheet is poor, for example, a urethane type adhesive, an
epoxy type adhesive, an acid-modified olefin resin adhesive, a copolyamide
type adhesive, a copolyester type adhesive or the like can be interposed
between the metal and the metal sheet.
In order to hide the metal sheet or assist the transmission of the blank
holding force to the metal sheet at the draw-redraw forming step, an
inorganic filler (pigment) can be incorporated into the coating or film
used in the present invention.
As the inorganic filler, there can be mentioned inorganic white pigments
such as rutile or anatase titanium dioxide, zinc flower and gloss white,
white extender pigments such as barite, precipitated barite sulfate,
calcium carbonate, gypsum, precipitated silica, aerosil, talc, fired or
unfired clay, barium carbonate, alumina white, synthetic or natural mica,
synthetic calcium silicate and magnesium carbonate, black pigments such as
carbon black and magnetite, red pigments such as red iron oxide, yellow
pigments such as sienna, and blue pigments such as ultramarine and cobalt
blue. The inorganic filler can be incorporated in an amount of 10 to 500%
by weight, especially 10 to 300% by weight, based on the resin.
FIG. 5 shows an example of the coated metal sheet preferably sued in the
present invention. Chemical conversion coatings 12a and 12b such as
chromate treated films are formed on both the surfaces of a metal
substrate 11, and an inner face coating 13 is formed on the surface to be
formed into the inner face of the can, through the chemical conversion
coating 12a. An outer face coating comprising a white coating 14 and a
transparent varnish 15 is formed on the surface to be formed into the
outer face of the can, through the chemical conversion coating 12b.
Referring to FIG. 6 illustrating the forming process of the present
invention, a coated metal sheet as mentioned above is punched into a disk
20 having a thickness t.sub.B at the punching step. Then, at the
subsequent drawing step, the disk is draw-formed into a shallowly drawn
cup 23 comprising a bottom 21 having a large diameter and a thickness
t.sub.B and a side wall 22 having a thickness tw'. At this drawing step,
the draw ratio defined by the following formula:
##EQU2##
is preferably in the range of from 1.2 to 1.9, especially preferably from
1.3 to 1.8. The thickness tw' of the side wall 22 is slightly larger than
t.sub.B.
Then, at the first redrawing step, the shallowly drawn cup 23 is subjected
to redraw forming by an apparatus as shown in FIG. 1.
As shown in FIG. 1, in the redrawing method of the present invention, the
redrawing punch 1, the redrawing die 2, the holding member 3 and the
annular working member 4 are used. Preferably, the diameter of the
redrawing punch 1 is 1/1.2 to 1/1.9 of the inner diameter of the
preliminarily drawn cup 5. If this requirement is satisfied, a sufficient
blank holding face can be secured. The top end face 2A of the redrawing
die 2 is formed as the die flat face, and the radius of the corner portion
2B exerting a tensile bending action on the cup 5 is adjusted to 1 to 20
times the sheet thickness, preferably 1 to 4 times the sheet thickness
when reduction of the thickness is especially intended.
In the holding member 3, the diameter is reduced in the lower end portion.
The diameter of the large-diameter portion is substantially equal to the
inner diameter of the cup 5. The inner face of the side wall of the cup
abuts against the large-diameter corner portion 3B and small-diameter
corner portion 3C of the holding member 3, and the radius R.sub.H of the
small-diameter portion 3C is the blank holder radius. This radius R.sub.H
is adjusted to 3 to 20 times the sheet thickness, preferably 4 to 12 times
the sheet thickness when reduction of the thickness is especially
intended.
The annular member 4 can be fixed to the top end face of the redrawing die
or be formed integrally with the redrawing die. Alternatively, the annular
member 4 can be arranged independently from the redrawing die. A working
portion 4A is formed on the annular member 4 and a tapered face is formed
at the upper part thereof. The diameter of the working portion 4A is
diminished by 1 to 6%, preferably 1 to 5%, of the diameter of the
preliminarily drawn cup 5. The face of the cup being tapered at the
diminishment of the diameter forms an angle .theta. to the central axis,
and it is preferred that the angle .theta. be 15.degree. to 45.degree.,
especially 20.degree. to 40.degree..
By using the above-mentioned apparatus, a redraw-formed cup 26 comprising a
bottom 24 having a thickness t.sub.B and a diameter smaller than that of
the shallowly drawn cup and a side wall 25 having a thickness tw" and a
height larger than that of the shallowly drawn cup is prepared. The side
wall 25 of the redraw-formed cup 26 is bent and elongated so that the
thickness tw" is smaller than t.sub.B and tw'.
In general, this redrawing process is conducted in a plurality of stages,
and if the redrawing is carried out in a plurality of stages, the
thickness of the side wall is reduced and the thickness of the entire side
wall is made more uniform. At the final n-th redrawing step, a
deep-draw-formed can 29 comprising a small-diameter bottom 27 having a
thickness t.sub.B and a higher side wall having a thickness tw'" is
obtained.
Incidentally, the present invention is applied to not only the so-called
sequential redrawing but also the so-called reverse redrawing. An
embodiment where the present invention is applied to the latter redrawing
is illustrated in FIG. 4.
It is preferred that the draw forming and redraw forming be carried out
after a lubricant has been coated on the coated metal sheet or the
preliminarily drawn cup. As the lubricant, there can be used liquid
paraffin, synthetic paraffin, edible oil, hydrogenated edible oil, palm
oil, natural waxes and polyethylene wax. The amount coated of the
lubricant is changed according to the kind of the lubricant, but it is
generally preferred that the amount coated of the lubricant be 0.1 to 10
mg/dm.sup.2, especially 0.2 to 5 mg/dm.sup.2. Coating of the lubricant can
be accomplished by spraying the lubricant in the melted state on the
surface of the metal sheet or cup.
The draw forming can be carried out at room temperature, but it is
preferred that the draw forming be carried out at a temperature of
20.degree. to 95.degree. C., especially 20.degree. to 90.degree. C.
Of course, the method of the present invention can be conducted at the
redrawing step in the process of the production of a draw-ironed can
(so-called DI can).
The obtained can is formed into a can barrel for a two-piece can through
various processings such as flange trimming, doming, necking and flanging.
According to the present invention, by arranging an annular working member
having a working portion having an inner diameter smaller than the outer
diameter of the side wall of the preliminarily drawn cup on the face of
the redrawing die, passing the preliminarily drawn cup through the working
member to reduce the diameter and carrying out redrawing by delivering the
cup to the portion of engagement between the blank holder face and the
redrawing die, a redraw-formed cup having a uniformly and sufficiently
reduced thickness can be obtained without damage of the metal sheet or
coating layer. Furthermore, since the bending-unbending operation is
carried out in advance before the redraw forming, work hardening is
controlled in the redraw-formed cup and therefore, post treatments of the
redraw-formed cup, such as necking, can be performed very easily.
The present invention will now be described in detail with reference to the
following examples.
EXAMPLE 1
An epoxy type thermosetting paint was coated and baked on both the surfaces
of a tin-free steel sheet (tempering degree of DR-9) having a blank
thickness of 0.18 mm to form a protecting coating having a dry thickness
of about 20 .mu.m on each surface, and palm oil was coated and the coated
sheet was punched into a disk having a diameter of 179 mm. The disk was
formed into a shallowly drawn cup between a drawing punch and a drawing
die according to customary procedures.
The draw ratio at this drawing step was 1.42.
At first, second and third redrawing steps, redraw forming was carried out
by using the apparatus shown in FIG. 1.
The redraw ratios adopted at the first, second and third redrawing steps
were as follows.
First redraw ratio: 1.29
Second redraw ratio: 1.24
Third redraw ratio: 1.20
At each redrawing step, the curvature (R.sub.D) of the working corner
portion of the redrawing die was 0.6 mm and the curvature radius (R.sub.H)
of the working corner portion of the holding member was 1.5 mm.
The ratio .delta. of the diminishment of the diameter by the annular member
and the tapering angle .theta. of the material at the diminishment of the
diameter were as follows.
______________________________________
.delta.
.theta.
______________________________________
First redrawing 3% 35.degree.
Second redrawing 4% 35.degree.
Third redrawing 5% 35.degree.
______________________________________
The blank holding load at the forming was 3,000 Kg.
The characteristics of the obtained deep-draw-formed cup were as follows.
Cup diameter: 66 mm
Cup height: 130 mm
Average thickness change ratio of side wall: -20%
Then, doming and trimming were carried out according to customary
procedures, and thin, necking was carried out by using an ordinary necking
die having an inner diameter smaller than the outer diameter of the cup.
No significant defects were found. When the Vickers hardness of the
portion close to the trimmed part of the cup before necking was observed,
it was found that the hardness HV was 225 while the hardness HV of the
starting sheet was 190, and it was confirmed that work hardening was not
conspicuously advanced.
Then, flanging was carried out, and degreasing and washing were then
conducted. Thus, a can barrel for a two-piece can was obtained.
In order to check damage of the protecting coating of the final can barrel,
the degree of the exposure of the metal was measured. The enamel rater
value was smaller than 0.1 mA.
The redraw-formed can was cold-filled with (A) cola, (B) beer or (C)
synthetic carbonated drink, and a metal lid was double-seamed to the
filled can. Then, the so-prepared three vessels were heat-sterilized under
conditions shown in Table 1.
TABLE 1
______________________________________
Spontaneous
Vessel Apparatus Temperature
Pressure
______________________________________
(A) can warmer 42.degree. C.
7.0 kg/cm.sup.2
(B) pasterizer 62.degree. C.
6.2 kg/cm.sup.2
(C) can warmer 42.degree. C.
8.0 kg/cm.sup.2
______________________________________
These three filled can vessels were stored at room temperature of
37.degree. C. for a long period, and the corrosion of the inner face of
the can was observed and the corrosion resistance was evaluated. The
obtained results are shown in Table 2. As is seen from the results shown
in Table 2, no trouble was found in any vessel, and interfacial corrosion
was not caused at all.
TABLE 2
__________________________________________________________________________
Storage Period
1 month 3 months 6 months
Content
Evaluation Item
corrosion
leakage
corrosion
leakage
corrosion
leakage
__________________________________________________________________________
Cola .largecircle.
0/100
.largecircle.
0/100
.largecircle.
0/100
Beer .largecircle.
0/100
.largecircle.
0/100
.largecircle.
0/100
Synthetic .largecircle.
0/100
.largecircle.
0/100
.largecircle.
0/100
Carbonated
Drink
__________________________________________________________________________
Note
.largecircle.: no corrosion
COMPARATIVE EXAMPLE 1
An epoxy type thermosetting paint was coated and baked on both the surfaces
of a tin-free steel sheet (tempering degree of DR-9) having a blank
thickness of 0.18 mm to form a protecting coating having a dry thickness
of about 20 .mu.m on each surface, and palm oil was coated and the coated
sheet was punched into a disk having a diameter of 179 mm. The disk was
formed into a shallowly drawn cup between a drawing punch and a drawing
die according to customary procedures.
The draw ratio at this drawing step was 1.42.
At first, second and third redrawing steps, redraw forming was carried out
by using the apparatus shown in FIG. 7.
The redraw ratios adopted at the first, second and third redrawing steps
were as follows.
First redraw ratio: 1.29
Second redraw ratio: 1.24
Third redraw ratio: 1.20
At each redrawing step, the curvature (R.sub.D) of the working corner
portion of the redrawing die was 0.4 mm and the curvature radius (R.sub.H)
of the working corner portion of the holding member was 1.3 mm.
The blank holding load at the forming was 3,000 Kg.
The characteristics of the obtained deep-draw-formed cup were as follows.
Cup diameter: 66 mm
Cup height: 130 mm
Average thickness change ratio of side wall: -20%
Accordingly, the size of the obtained cup was substantially the same as
that of the cup obtained in Example 1.
In order to check damage of the protecting coating of the final can barrel,
the degree of the exposure of the metal was measured. It was found that
the enamel rater value of the entire vessel was 10 mA, the enamel rater
value of the upper part of the side wall was 7 mA and the enamel rater
value of the lower part of the side wall was 3 mA. Accordingly, it was
confirmed that the protecting coating at the upper part of the side wall
was considerably damaged.
Then, doming and trimming were carried out according to customary
procedures, and then, necking was carried out by using an ordinary necking
die having an inner diameter smaller than the outer diameter of the cup.
Many wrinkles were formed in the worked portion and buckling was caused in
an extreme case.
When the Vickers hardness of the portion close to the trimmed part of the
cup before necking was observed, it was found that the hardness HV was 240
while the hardness HV of the starting sheet was 190, and it was confirmed
that work hardening was conspicuously advanced.
EXAMPLE 2
A coated metal sheet was prepared by bonding a polyethylene terephthalate
(PET) film (having a thickness of 20 .mu.m, a glass transition temperature
of 70.degree. C. and a melting point of 255.degree. C.) to both the
surfaces of a tin-free steel (TFS) sheet having a blank thickness of 0.18
mm and a tempering degree of DR-9.
The coated steel sheet was redraw-formed in the same manner as described in
Example 1. A can barrel having substantially the same shape as that of the
can barrel obtained in Example 1 was obtained.
The obtained deep-draw-formed can was washed and heat-treated, degreased
and washed according to customary procedures. Then, trimming, printing
(baking at 205.degree. C. for 2 minutes), necking and flanging were
conducted.
In order to check damage of the protecting coating of the final can barrel,
the degree of the exposure of the metal was measured. The enamel rater
value was smaller than 0.1 mA.
Then, the adhesion strength of the polyethylene terephthalate film as the
coating material was measured according to the method in which the barrel
portion was cut out in a width of 5 mm along the can height direction from
the obtained final redraw-formed can barrel and 90.degree. peel strength
of the film was determined.
It was found that the adhesion strength was 0.17 to 0.56 kg/mm and there
was no practical problem.
COMPARATIVE EXAMPLE 2
The coated metal sheet used in Example 2 was redraw-formed in the same
manner as described in Comparative Example 1, whereby a can barrel having
substantially the same shape as that of the can barrel obtained in Example
1 was obtained. Peeling deemed to be due to reduction of the adhesive
force was caused at parts of the coatings on the inner and outer faces of
the can barrel, and this peeling was especially conspicuous in the upper
portion of the can barrel. Accordingly, the subsequent can-manufacturing
operations became impossible.
EXAMPLE 3
A bright tin-deposited steel sheet having a blank thickness of 0.29 mm and
a tin deposition amount of #25/#25 (the tempering degree was T-2.5) was
punched into a disk having a diameter of about 145 mm, and the disk was
formed into a cup having a diameter of about 80 mm by a drawing punch and
a drawing die according to customary procedures.
Redraw forming of this cup was carried out by using the apparatus shown in
FIG. 1. The redraw ratio adopted was 1.21. The diameter diminishment ratio
.delta. was 3.5% and the tapering angle .theta. was 30.degree.. The
obtained cup having an inner diameter of about 66 mm was subjected to
three-staged ironing between a punch and an ironing die. The obtained can
barrel had an excellent dimensional precision. There was no problem about
the surface properties on the outer face side.
EXAMPLE 4
A laminated sheet was prepared in the following manner.
A cold-rolled steel sheet strip having a thickness of 0.30 mm, a tempering
degree of T-2.5 and a width of 300 mm was subjected to a known
electrolytic chromate treatment, whereby a coating comprising 0.017
g/m.sup.2 as chromium of an upper chromium oxide hydrate layer and 0.10
g/m.sup.2 of a lower metallic chromium layer was formed on one surface of
the steel sheet strip. then, 5.6 g/m.sup.2 of tin was deposited on the
other surface of the steel sheet strip. Then, the surface-treated steel
sheet strip was heated at 220.degree. C. by using a roll heater, and a
biaxially oriented polyester film (polycondensate of ethylene glycol with
80% of terephthalaic acid and 20% of isophthalaic acid) having a thickness
of 25 .mu.m was laminated on the surface having the chromium oxide hydrate
layer and immediately, the laminated steel sheet strip was water-cooled.
The obtained polyester resin-coated steel sheet was subjected to
draw-ironing under the same forming conditions as described in Example 3
so that the polyester resin-coated surface became the inner face of the
obtained DI can.
The obtained can barrel had an excellent dimensional precision, and there
was no problem about the surface properties on the outer face side.
Furthermore, the coating on the inner face side had no defect.
Top